Electrical control system



Feb. 18, 1947. P. HALPERT ETAL 2,415,819

ELECTRICAL CONTROL SYSTEM Original Filed Aug. 1 9, 1941 2 Sheets-Sheet l TEL- 5- I I INVENTORS R HALPEIPT, CfAfkuwaz.

Feb. 18, 1947. P. HALPERT ET AL ELECTRICAL CONTROL SYSTEM Original Filed Aug. 19, 1941 2 Sheets-Sheef2 dffswu reiented Feb. 18,1947

UNITED STATES PATENT OFFICE ELECTRICAL CONTROL SYSTEM Continuation of application Serial No. 407,460, August 19, 1941. This application filed June 22, 1942, Serial No. 448,040

1'7 Claims. (Cl. 172-239) This application is a continuation of copending application Serial No. 407,460, for Electrical control systems, filed August 19, 1941, which has been replaced by this and other divisional or continuation applications including Serial No. 448,794, filed June 27, 1942.

This invention relates to positional control systems, for remotely controlling and accurately positioning objects such as searchlights and guns, from an accurately positioned or position-maintaining instrument. By our invention we may also control the steering of a dirigible vehicle, such as ship or airplane, by positioning the rudder or other control surface. Our invention also has application to straight follow-up systems wherein the position of a. follow-up element is accurately maintained with reference to a rotatable control or sensitive element such as a gyro compass or directional gyro.

For this purpose, we provide a novelpick-ofi device for producing an electrical signal in response to displacement of the control or sensitive element which reverses phase upon reversal of the displacement. In former pick-off devices of this type, for example that shown in Patent No. 1,959,804, to Wittkuhns et al., issued May 22, 1934, for Non-contacting follow-up system, a useful signal is secured only through a comparatively small angle of deflection of the control element, say degrees or less. We overcome this limitation of the prior art and provide a pick-oil means in which a positive signal is generated for angular deviations approaching 90 degrees. In addition, we may provide a pick-off device in which the signal output continuously increases as the deviation increases, rather than one which quickly reaches a maximum constant signal, as is the case in therabove-mentioned patent.

It has been recognized that the generation of a simple reversible signal is not sufiicient for control purposes even where such signal is proportional to the relative displacement between the position-maintaining device and the controlled device. For obtaining accurate control, we propose to generate an electrical signal which not only varies in phase and magnitude with the direction and amount of relative displacement between the control and controlled device but which also varies with the relative velocity and preferably also a higher time derivative of the relative displacement. This is done by operating electrically in our improved rate amplifier upon the signal obtained from our improved pick-off means so as to add to this signal certain components representing the velocity and acceleration of the displacement of the controlling object. With this improved system, it is possible to maintain the relative displacement between two objects at practically zero, or to control the position of a remote object with very high accuracy.

Specifically, we employ a Ward-Leonard type of control using a motor-generator set, the generator having two opposed field windings. The A.-C. reversible phase control signal obtained from our improved pick-off means is amplified and modified by the addition of velocity and acceleration components in our improved rate amplifier, whose output includes two direct currents which excite the opposed generator field windings to produce one or the other direction of control of the controlled object, in accordance with the phase of the pick-off signal. The controlled object may be a follow-up motor or a servo-motor.

Included in our improved amplifier are means for permitting direct coupling of D.-C. amplifier stages which are A.0. energized. This may be done by balancing out the fixed bias produced in the output of one stage to prevent its affecting the input of the next stage, and simulaneously providing a signal hold-over means, from one half-cycle of applied energy to the next halfcycle, whereby successive stages may operate successfully despite opposite phase of energization.

The present application constitutes a continuation-in-part of prior co-pending application now Patent No. 2,398,421 for Electro-hydraulic control system, dated April 16, 1946, in the names of Carl A. Frische, George P. Bentley, and Percy Halpert, and assigned to the same assignee as the present application, the first and last of these inventors being also co-inventors in the present case.

Referring to the drawings:

Fig. 1 is a perspective view of the pick-01f or signal generating portion for a positional control device, such as used with a directional gyro or a gyro compass.

Fig. 2 is a similar view of a slightly modified form of signal generator.

Fig. 3 is a, wiring diagram showing a modified Ward-Leonard servo system, and the rate amplifier therefor, controlled from the signal generator of Fig. l or Fig. 2.

The pick-off means or signal generator is shown in Fig. 1 as a variable inductive device comprising a three-legged transformer l mounted on a gear follow-up platform 2 which may be rotated from the follow-up motor 3. The center leg 4 of the transformer is shown as continuously excited from a single phase A. C. source 2| while the two outer legs and 6 are connected in opposition so that their output through lines 11' is zero when the transformer is balanced, but is of one phase as the induced current in one leg predominates and of the other phase a the induced current predominates in the other leg. Cooperating with said transformer is shown a pair of soft iron armatures 8 and 9 which may be of nearly semi-circular form, each mounted on a common non-magnetic base III secured to vertical stem II rising from the vertical ring I2 of a gyroscope I4 through an aperture in platform or gear 2 and the hub I5 of the support for transformer I, so that said armatures are fixedly oriented in azimuth from the gyroscope I l. The armature 8 is shown as of less diameter than armature 9 so that it passes between and above poles 4 and 6 while the armature 9 passes between and above poles 4 and 5. In the normal balanced condition, the two ends I3 and I3 of the armatures lie about on a line with the two outer edges of the poles I, 5 and t, in which position there is no net output from the signal generator through lines 1-1, since the voltages induced in legs 5 and 6 then balance one another.

Preferably each pole piece is tapered, gradually increasing in cross-sectional area as it recedes from the tips I3 and I3. By this means, a signal which increases with angular displacement is secured through a fairly large displacement angle, as opposed to the prior art as indicated by the above cited patent. This form of inductive pick-01f also has the advantage that control is not lost except when the angular displacement approaches 90 either side of the balanced position, thus maintaining full control over 180 degrees of arc.

The output from this pick-off device, which is the voltage appearing across terminals 1-1, may be amplified in the rate amplifier shown in Fig. 3, which, as will be later described, introduces Velocity and, acceleration components into the control signal, which then may control motor 3 to drive follow-up platform 2 and transformer I into correspondence with armatures 8, 9, the system then acting as a follow-up system. The position of platform 2, which is thereby kept synchronized with that of ring I2 and hence with gyroscope It, may be transmitted to a remote point as by Selsyn transmitter I6, to give a suitable remote indication.

Alternatively, the output of the rate amplifier may control any suitable remote motor, such as 241 (Fig. 3) to remotely control any suitable object, such as a rudder 25I. The system may thereby act as an automatic pilot, turning a craft until the orientation of platform 2, which would then be fixed to the craft, correspond with that of gyroscope I4. The craft then acts as the follow-up element of the system.

It will be obvious that the mountings of transformer I and armatures 8, 9 may be interchanged; that is, the transformer may be mounted on the gyro vertical ring 12, and the armatures on the platform 2.

Also armatures 8, 9, instead of tapering from both ends to the center, might taper continuously from one end to the other. Thi would In this case, the center pole I is of greater width than the outer poles 5' and 3' and the gyro stabilized armatures 8' and 9 are shown as of uniform width. With this construction, armature 8 effectively cooperates only with poles I and 5', while armature 9' cooperates only with poles I and 6.

With this type of pick-01f, a sharp increase in signal output is obtained for small displacements of armatures 8', 9' relative to transformer I, but the signal does not vary with displacement over a wide angle, as is the case with the device of Fig. 1. However, full control is maintained over the entire 360 degrees of rotation.

Referring now to Fig. 3, the signal output from the pickoff unit I is fed to primary winding I03 of input transformer I65 by wires I01. Transformer I05 has a center tapped secondary winding I09 and two other similar secondary windings III and H3. Center tap I15 of winding I68 is connected to the cathodes II1, N9 of duplex tube I2I by way of cathode bias resistor I23 whose resistance value is selected so as to permit both sections of tube I2I to operate at the center of linear portions of their characteristic curves. Tube IZI is indicated as being a twintetrode having a pair of control grids I25, I21, a pair of additional grids I29, I3I and a pair of anodes I33, I35.

Control grids I25, I 2'? are connected to the outside terminals of secondary I09, thereby being energized in push-pull fashion. Anodes I33, I35 are energized cophasally from a source I31 of alternating current, which also supplies the pickoff unit I, by way of conductor I39. The other terminal of source I31 is connected by wire M9 to center tap N5 of secondary winding I09. Grids I29, I31, which act a screen grids in the present illustration, are energized from source I31 by way of a voltage dropping resistor I5I. Bypass condensers I53, I55, I51, I59 are also used.

In the absence of input voltage to grids I 25, I21, the circuits of both sections of tube I2I are balanced, so that equal (and opposite) voltages will appear across output resistors I II, I43.

In operation, the output of pickoff device I will be an alternating voltage having the same or opposite phase with respect to that of source I31. Phase adjuster I86, which may be a variable condenser, is adjusted to assure this proper phase relation. This phase adjuster may be inserted in lines 2 I, if desired. The output voltage is fed in push-pull to the control grids I25, I21 by way of transformer H15. Hence, one of the two grids I25, I21 will have impressed Voltage of the same phase as its corresponding anode, and the other will have opposite phase, it being noted that both anodes I33, I35 are energized in like phase from source I31. Which particular grid I25 or I21 has this same phase as its corresponding anode will depend on the phase of the output voltage from pickoff device WI, and will, therefore, depend on the sense of the relative motion between the armature IBI and field I63 of the pickoff device I. For purposes of illustration, let it be assumed that grid I25 has a voltage impressed on it in phase with its anode I33. Then, during the half-cycles of supply source I31 when anode I33 is positive with respect to cathode I I1, the upper section of tube I3! will be more conductive than with zero voltage applied to grid I25, and an increased output voltage will appear across output resistor I4 I. During the other half-cycles, the conduction of this section of tube I2I will be blocked by its anode voltage, which is then negative with respect to cathode II 1. The upper section of tube I2I, therefore, acts as a half-wave rectifier. Tuned choke I45 and bypass condensers I53, I55 prevent the alternating component of the output voltage of tube I2! from appearing across output resistor I II.

At the same time, in the illustrative example used, grid I21 will have a negative voltage applied to it during the positive anode half-cycle, so that the current in, and the voltage drop across, resistor I43 will decrease from the value with zero voltage on grid I21. The net change in direct voltage across resistors MI and I43 corresponds in magnitude to the magnitude of the alternating voltage output from pickoff device I, and in sense to the phase of that voltage, and thereby to the relative deviation of the pickoif armature. Tube I2I, therefore, acts as a phase sensitive amplifier and half-Wave rectifier for the signal voltage from the pickofi device I.

Connected across output resistor MI is a differentiating circuit comprising condenser I65 and variable resistance I61. A similar differentiating circuit I69, I1I is connected across the other output resistor I43.

Since condenser I85 (or I69) will pass current only if the voltage applied from resistor IM (or I43) is varying, it will be evident that this current corresponds to a rate of change, or time derivative, of the voltage output of tube I2 I, and therefore, of the voltage output of pickoff I. This rate current, flowing through resistor I61 (or I1I) will produce a rate voltage, adjustable in magnitude by adjusting the resistance value of resistor I61 (or I1I). This rate voltage is applied to a similar diiferentiating circuit I13, I

to produce a voltage across resistor I15 corresponding to acceleration; that is, the time derivative or rate of change of the rate voltage. In order that the voltage across resistor I15 may have a rate component in addition to its acceleration voltage, variable resistor I19 is shunted across condenser I13, thereby permitting an adjustable component of rate current to flow through resistor I15. A similar circuit IBI, I83, I85 is placed in the lower section of the push-pull circuit. Adjusting resistor I51 (or III) will adjust the amount of rate voltage produced, and will, therefore, also adjust the acceleration voltage produced, since the second differentiating circuit I13, I15 (or I8I, I83) depends for its input on the voltage across resistor I61 (or I1I). Adjusting resistor I19 (or I85) adjusts the relative amounts of rate and acceleration components produced across resistor I15. This may be readily seen from the fact that with a zero value resistance I19, no diiferentiation is possible, and the output is rate only. With infinite resistance value for I19, no velocity component is transmitted, and the output is pure acceleration. Intermediate value of resistance, of course, provide varying relative amounts of rate and acceleration components.

In each of these differentiating circuits, the resistance (I61, I15, I1I, I83) is made small in value compared to the reactance of its corresponding capacitance (I65, I13, I69, IBI) at the low frequencies corresponding to the rates of variation of signal voltage magnitude, in order to provide effective difierentiation. Also, the total impedance of the second differentiating circuits (I13, I15 and I8I, I83) is made high compared to that of resistance I61 or III, in order to prevent undue loading of the first differentiating circuits (I65, I61 and I69, I1I).

In the same manner as the second derivative or acceleration voltage is obtained, third, fourth or higher derivatives, as desired, may also be obtained.

The combined rate and acceleration voltage appearing across the resistance I15 (or I83) is connected in series with a voltage representing the original signal or displacement voltage itself, obtained from secondary winding III (or II3) by means of magnitude adjusting potentiometer I81 (or I89). Winding III, in the illustrative example used, is connected so as to increase the current to anode 20I, while winding II 3 is connected oppositely. The combined displacement, rate, and acceleration voltage is applied between cathode I9I (or I93) and control grid I95 (or I91) of tube I11, in series with cathode biasing resistor I99. Anodes 2IlI, 203 of tube I 11 are energized through their respective load resistors 265, 201 from source I31. It will be noted that one terminal of source I31 is already connected, by wire I39 and resistor I99 to cathodes I9I, I93. Hence, wire 209 leading to anodes 20I, 203 is connected to the other side of supply source I31. Grids 2| I, 2I3 act as screen grids and are connected to wire 209 by voltage dropping resistor 2 I 5.

Anode 20I is connected directly to control grid 2I9 of duplex tube 223, and similarly anode 203 is connected directly to grid 22 I. Anodes 29I and 203 are connected together by condenser 2 I 1. The common terminal 225 is connected to cathodes 221, 229 by cathode biasing resistor 23I.

The anodes 233, 235 are energized from source I 31 through their respective matched loads 231, 239 which form two similar and opposed field windings for direct current generator 24 I. These windings are bypassed by bypass condensers 293, 245. Since cathodes 221, 229 are directly connected to wire 269, anodes 233, 235 must be connected to the other terminal of source I31, as by conductor I39.

The field windings 231, 239 form highly inductive loads, which, when working out of low resistance driving sources, such as power amplifier tubes or gas-filled tubes, would have high time constants, preventing close following of the con trolled motor. However, in the present system the generator fields are matched to the plate resistances of their respective amplifier tubes, and the combination is one inherently having a small time constant, permitting accurate control.

Tube I11 serves as a D.-C. amplifier for the combined displacement, rate and acceleration voltage connected to its control grids. Since this tube is energized by alternating current from source I31, it will conduct only during half cycles in which conductor 299 is positive with respect to cathodes I9I, I93. Hence, voltage will appear across its output resistor 265 (or 291) only during these half-cycles. The circuits of both sections of tube I11 are also balanced, so that, with zero signal on both grids I95, I91, equal cursrent will flow in, and equal voltage drops will appear across, output resistors 265, 291. Cathode biasing resistor I99 is selected in resistance value to provide operation about the center of the linear portion of the grid characteristic of each section of the tube I11.

It will be seen that, during those half cycles when tube I11 conducts, the total voltage drop across resistor 265 (or 291) will bias grid 2I9 (or 22I). However, this will not affect the output of tube 223, since during the same half-cycles, anodes 233, 235 are negative and tube 223 does not conduct. During the other half cycles, no grid voltage would appear on grids M9, 22!, as tube H! is then non-conductive. To avoid this difficulty, condenser 2 I! is connected across grids 2!!) and 22!, being also across output resistors 265, 207. With zero input to tube I'll, condenser 2!! is uncharged, since the stand-by or zero input, voltage across,resistor 205 balances and cancels that across resistor 2B1, leaving zero net voltage across condenser 2!!. When input voltage is applied to the grids !95, I9"! of tube I'll, the voltages across resistors 265, will change, and in opposite directions, during half-cycles when anodes 20!, 263 are positive. In the illustrative example used, the voltage across resistor 265 will increase. and that across 20! will decrease. The net change in voltage will charge condenser 2!l, so that, during the succeeding half-cycles, when tube H7 is non-conductive, this net voltage Will persist and be maintained on grids 2l9, 22!, and will control tube 223, since anodes 233, 235 are then positive. In this manner, the stand-by voltages across resistors 265, 20'! have no effect on grids 2l9, 22!, only the changes in these voltages being used in the succeeding stage. This arrangement provides a very successful direct-coupled direct-current amplifier directly energized from a single source of alternating current, since the standby voltages have no effect on the su ceeding stage.

Condenser 2!! may be made adjustable, and should be adjusted to give maximum output.

It is clear that the number of D.C. amplifier stages may be extended as much as desired, merely assuring that each stage is energized from the A.-C. source in a phase relation opposite to that of the previous stage, and that some means, such as condenser 2! l, is used to enable holdover of the useful portion of the output voltage from the preceding stage, and to prevent the adverse biasing effect of the standby voltage drops.

The output currents of tube 223 are smoothed by condensers 243 and 225 and energize field windings 231 and 239 of generator 24!, which form the load on, and are matched in impedance to, the sections of tube 223. Windings 23! and 239 are identical, and the sections of tube 223 are balanced so that equal currents will flow in windings 231, 239 with zero input to tube 223. Tube 223 is also operated at the center of the linear portion of the grid characteristic. Under these conditions the effects of the two windin s 237, 239 cancel one another. When input voltage is applied to the grids 2l9, 22! of tube 223, in the manner described above, the current through one Winding 23'! or 239 will increase and that through the other will decrease, producing a net magnetic field whose magnitude and polarity correspond to the magnitude and polarity of the signal. Generator armature 24! is driven at constant speed by any suitable means, such as motor 242, and will generate a voltage also corresponding'it'o the combined signal. This generator output voltage is fed to the armature of direct current motor 241, whose field 2 19 has fixed excitation from a suitable direct current source. The armature resistance of motor 247 is matched to that of the generator 24!, includin the effect of armature reaction, so as to provide maximum power transfer therebetween. Hence, motor 247 will turn at a speed corresponding to the combined voltage and in a direction corresponding to the signal voltage phase. Motor 24'! may be directly coupled, as by pulley arrangement 2 38, to any type of load, such as rudder 25!, or follow-up device 2.

Tubes 2!, l1! and 223 have been illustrated as being of the duplex or twin type of tube. It is obvious that two separate tubes could be used in place of each of these twin tubes. Also, these tubes need not be tetrodes or triodes, as shown. It is immaterial whether they be triodes, tetrodes, pentodes or other types of tubes, so long as they operate as linear amplifier tubes. Also, they may be of the vacuum or grid-controlled gas filled type.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings sha11 be interpreted as illustrative and not in a limiting sense.

Having described our invention, what we claim and desire to secure by Letters Patent is:

1. In an automatic positional control device, the combination with the control and controlled device, of means including an inductive electric signal generator for producing an alternating displacement signal whose magnitude varies with the relative displacement of said devices and whose phase varies with the sense of such displacement, an amplifier circuit connected to said signal generator, means in said amplifier for obtaining both the first and higher time derivatives of said signal, means for amplifying a combination of said displacement and time derivative signals and producing therefrom a pair of D. C. outputs, an opposed field generator having one field excited from each of said outputs, and a motor for turning said controlled device having its armature electrically connected to the armature of said generator.

2. An automatic position maintaining device comprising a position-maintaining instrument, pick-off means for generating a signal corresponding to relative displacement between said instrument and said device, means for producing a signal corresponding to said relative displacement and time derivatives thereof, push-pull direct coupled means for amplifying said produced signal to obtain two D.-C. output currents whose difference corresponds in polarity to the sense of said displacement, a generator having two opposed field windings matched to the outputs of said amplifying means, means for energizing said field windings respectively by said two currents, a motor for orienting said device, and means for controlling said motor from the output of said generator,

3. An automatic positional control device as in claim 1, in which said signal producing means comprises a transformer mounted on one of said devices and having two separate open magnetic circuits, an output coil linking each of said circuits, means for supplying an alternating magnetomotive force to each of said circuits, a pair of magnetic armature members, means for mounting said members on the other of said devices in such a manner that each member cooperates with one of the gaps of said circuits in a differential manner with respect to the other member, and means for connecting said coils in opposition whereby an output voltage having a phase sensitive to the sense of said relative displacement is obtained.

4. An automatic positional control device as in claim 1, wherein said signal generator comprises a transformer having three legs, a coil wound about each leg, means for energizing the coil of one leg with alternating current, means connecting said other coils to form an output circuit, means for maintaining said transformer on one of said devices, a pair of arcuate magnetic armatures, means for supporting said armatures on the other of said devices in such manner that one armature partially completes the air gap for flux passing between said energized leg and one of said other two legs and said other armature partially completes the air gap for flux passing between said energized leg and the other of said other two legs, whereby said relative displacement will differentially change said air gaps and there-- fore the voltage induced in each of the other of said windings.

5. An automatic positional control device as in claim 1, wherein said combination signal amplifying means comprises two electron discharge tubes, means connecting the output of one tube directly to the input of the other tube, a source of alternating current, and means for energizing said jtgpbes in opposite phase from said source.

6. An automatic positional control device as in claim 1, in which said combination signal amplifying means comprises an alternating current energized direct-coupled direct current amplifier comprising a first pair of electron discharge tubes connected in push-pull fashion, an output resistor connected to the output of each tube, a source of alternating current, means for energizing said tubes cophasally from said source through their respective resistors, a second pair of electron discharge tubes, means connecting the input circuits of said second pair of tubes directly across the respective output resistors of said first pair of tubes, means for energizing said second pair of tubes from said source in phase opposition to the energization of said first pair of tubes, and means for carrying over the voltage across said resistors from one half-cycle of said source to the succeeding half-cycle.

7. An automatic positional control device as in claim 1 wherein said amplifier circuit and said derivative signal obtaining means comprise a rate amplifier having means for producing a second signal corresponding to a time derivative of said displacement signal, means for adjusting the magnitude of said second signal, means for producing a third signal from said second signal and corresponding to a combination of a time derivative of said adjusted second signal and of said actual second signal, means for adjusting the relative proportions of said two signals composing said combination, and means for adding to said adjusted combination an adjustable signal corresponding to said original signal.

8. An automatic positional control device as in claim 1, in which said amplifier circuit and said derivative signal obtaining means include a rate amplifier comprising means for producing a second signal corresponding to a time derivative of said displacement signal, means for producing from said second signal a combination of signals corresponding to time derivatives of said displacement signal, means for varying the relative proportion of the signals comprising said combination, and means for adjusting the magnitude of said second signal whereby all the signals of said combination may be varied simultaneously.

9. In an automatic positional control device,

the combination, with the controlling and controlled devices, of means for producing a displacement signal corresponding to the relative displacement between said devices, means for obtaining both the first and higher time derivatives of said signal, means for producing a pair of direct current outputs from a combination of said displacement and time derivative signals, an opposed field generator having one field excited from each of said outputs and a motor for turning said controlled device having its armature electrically connected to the armature of said generator.

10. In an automatic positional control device, the combination, with the controlling and controlled device, of means for producing a displacement signal corresponding to the relative displacement between said devices, means for obtaining at least one time derivative of said signal, means for producing a pair of direct current outputs from a combination of said displacement and derivative signals, an opposed field generator having one field excited from each of said outputs and a motor for turning said controlled device having its armature electrically connected to the armature of said generator.

11. In an automatic positional control device, the combination, with the controlled and controlling devices, of means including an inductive electric signal generator for producing an alternating displacement signal whose magnitude varies with the relative displacement between said devices and whose phase varies with the sense of such displacement, an amplifier circuit connected to said signal generator, means in said amplifier for obtaining both the first and higher time derivatives of said signal, means for amplifying a combination of said displacement and time derivative signals and producing therefrom a D. C. output, a generator having a field excited from said output and a motor for turning said controlled device having its armature electrically connected to the armature of said generator.

12. In an automatic positional control device,

the combination, with the controlling and controlled devices, of mean for producing a displacement signal corresponding to the relative displacement between said devices, means for obtaining at least one time derivative of said si nal, means for amplifying a combination of said displacement and said time derivative signals and producing therefrom a direct current output, a generator having a field excited from said output and matched thereto and a motor for turning said controlled device having its armature electrically connected to the armature of said generator.

13. An automatic position maintaining device comprising a controlled object, a position maintaining instrument, pick-off means for generating a signal corresponding to relative displacement between said instrument and said object, means for producing a signal corresponding to said relative displacement and a time derivative thereof, means for obtaining from said signal two direct current outputs whose difierence corresponds in polarity to the sense of said displacement, a generator having two opposed field windings matched to said outputs, means for energizing said field windings respectively by said outputs, a motor for orienting said object, and mean for controlling said motor from the output of said generator.

14. An automatic pilot for dirigible craft, comprising a direction-maintaining instrument, pick-off means for generating a signal corresponding to relative displacement between said instrument and the attitude of said craft, means for producing a signal corresponding to said relative displacement and time derivatives thereof, push-pull direct-coupled means for amplifying said produced signal to obtain two uni-directional output currents Whose difierence corresponds in polarity to the sense of said displacement, a generator having two opposed field windings matched to the outputs of said amplifying means, means for energizing said field windings respec tively by said two currents, a motor for controlling said craft, and means for controlling said motor from the .output of said generator.

15. An automatic pilot for dirigible craft, comprising a direction maintaining instrument, pickofi means for generating a signal corresponding to relative displacement between said instrument and the attitude of said craft, means for producing a signal corresponding to said relative displacement and a time derivative thereof, means for obtaining two direct current outputs from said signal, the difierence of said outputs corresponding in polarity to the sense of said displacement, a generator having two opposed field windings respectively energized by and matched to said outputs, a motor for controlling said craft, and means for controlling said motor from the output of said generator.

16. An automatic pilot for dirigible craft having a control surface comprising a directionmaintaining instrument, pick-off means for generating a signal corresponding to relative displacement between said instrument and the attitude of said craft, means for producing a signal corresponding to said relative displacement and a time derivative thereof, means responsive to said produced signal for producing a pair of output currents whose difference corresponds to the magnitude and sense of said displacement, motive means for controlling said control surface, and means for controlling said motive means in response to said pair of currents.

17. An automatic pilot for dirigible craft comprising a direction-maintaining instrument, means for producing a signal corresponding to the relative displacement between said instrument and the attitude of said craft, means for producing a signal correspondin to said relative displacement and a time derivative thereof, means for amplifying a combination of said displacement and time derivative signals and producing therefrom a direct current output, a generator having a field winding matched in impedance to the output of said amplifying means and excited from said output, and a motor for controlling said craft having its armature electrically connected to the armature of said generator.

PERCY HALPERT. CARL A. FRISCHE. JOHN L. BIRD. ORLAND E. ESVAL 

